Age hardening of stainless steels with niobium silicides



United States Patent 3,029,171 AGE HARDENING 0F STAINLESS STEELS WITH NIOBIUM SILICIDES William Rostoker and Clarence J. Carter, Chicago, Ill.,

assignors, by mesne assignments, to Atlas Steels Limited, Welland, Ontario, Canada, a corporation of Canada No Drawing. Filed Mar. 23, 1959, Ser. No. 800,926 7 Claims. (Cl. 148-38) This invention relates to stainless steels and more particularly to a method of age-hardening stainless steel and to the resultant age hardened alloy.

The full development of the potential market for stainless steel in the aircraft, consumer appliance and related fields requires stainless steel which in its as-quenched condition, has a suiliciently low hardness and high ductility for the steel to be worked readily in strip or sheet form and which can be age-hardened to provide sufficient hardness for adequate strength.

Stainless steels have previously been age-hardened using various carbides, aluminides and titanides. Titanium-aluminum, nickel-aluminum, and copper are a few of the many examples of precipitant that have been used for age-hardening chromium-nickel stainless steels. Similar precipitants have also been used in stainless steel in which part or all of the nickel has been replaced with manganese. The precipitants which have been referred to above each have various disadvantages such as hot shortness, lack of ductility in the as-quenched condition, lack of adequate hardness after age-hardening, poor welding characteristics and in some cases the precipitant causes depletion of the austenitic or ferritic matrix of chromium to an extent such that the corrosion and oxidation resistance sufier.

The object of this invention is to provide a new method of age-hardening stainless steel of the chromium, chromium-nickel, chromium-manganese or chromium-nickelmanganese type to give a stainless steel which combines various desirable properties as will be apparent from this description which follows.

A further object of this invention is to provide an agehardened stainless steel of the chromium, chromiumnickel, chromium-manganese or chromium-nickel-manganese type which has a combination of desirable properties.

Another object of this invention is to provide an agehardened stainless steel of the type referred to above in which age-hardening is achieved by the use of additives which are inexpensive and easy to handle.

Our invention involves the use of niobium-silicide as the age-hardening precipitant. The niobium may be added in any convenient form such as ferro-niobium. Similarly the silicon may be added in any convenient form such as ferro-silicon.

Niobium is used in an amount of 1.0-4% and silicon in an amount of 1.5-5 The sum of the niobium and silicon additions should be between 3% and 7%. The ratio of niobium to silicon should be from 1:3 to 2:1. The silicides which are formed have the formulae NbSi Nb Si Additions in ratios substantially outside these limits would result in excess niobium or silicon residual in the austenite which would have the detrimental effect of reducing ductility. The best results where formability is important, namely low as-quenched hardness and com- 3,029,171 Patented Apr. 10, 1962 ICQ bined with high hardness after aging are obtained with 1.54% niobium and 35% silicon. The best ratio is about 1:2 of niobium to silicon and particularly good results are given by about 2% niobium with about 4% silicon. While low as-quenched hardness is important in the case of rolling stock, the higher as-quenched hardness characteristic of other formulations in accordance with this invention may be acceptable or even preferred for utilizations where formability is less important. Thus where machinability and dimensional control are dominant requirements relatively high as-quenched hardness may be preferred to obtain the minimum dimensional change after machining. Similarly high as-quenched hardness is acceptable in the case of casting alloys.

The stainless steels with which this invention is concerned are chromium, chromium-nickel, chromiumnickel-manganese or chromium-manganese steels having 10-20% chromium, 0-10% nickel and 020% manganese where the sum of the nickel and manganese are not in excess of the following:

Percent Mn The structure of the steel produced in accordance with this invention is believed to be predominantly ferritic. A typical microstructure of an alloy in accordance with this invention appears to show a ferritic matrix with small occlusions of austenite.

It is essential that the steels have a low carbon content as otherwise the niobium preferentially attaches to the carbon instead of forming a silicide. The carbon should therefore be less than 0.08%. Other elements which should be avoided are oxygen and nitrogen each of which interferes with the formation of the desired silicides. Oxygen should be less than 0.05% and preferably less than 0.01% and nitrogen should be less than 0.05% and preferably less than 0.005%. Other impurities usually found in steel, such as phosphorus, sulphur and copper, may be present in the usual incidental amounts. It is permissible to include small amounts of alloying elements for special purposes, such as COITOSlOl'l resistance or other properties, by adding molybdenum, tungsten or vanadium in amounts of up to about 5%. The alloying element which is added should, however, be one which does not compete to any substantial extent with niobium to form silicides.

The aging temperature may range from about 800 F. to 1400 F. If the temperature is too low, hardening does not occur in a reasonable time. if the temperature is too high, no peak hardening is achieved and carbide precipitation is encouraged. It is preferred that aging be conducted at a temperature at which substantially the maximum hardness is developed. The time required for peak hardening varies from /2 to 10 hours or more, according to the amounts of niobium and silicon and the temperature used for aging.

The following alloy is particularly eitective in providing low as-quenched hardness and high aged hardness, namely 15-20% chromium, 04% nickel, 12-20% manganese Percent Mn l.5%-3% niobium and 35% silicon. For example an 15 EXAMPLE 1 The analysis of the specimens used for the purpose of the example was:

Preclpitating Agent Specimens of /2 in. x 1 in. x 1 in. were machined from forged bars, solution treated at 2200 F. for one hour, water quenched and aged at 600 F., 800' F., 1000 F. and 1200 F. for various times. The results are shown in Table I.

Table I 4 EXAMPLE 2 Various chromium nickel, steels were investigated using the same techniques as those described in connection with Example 1 with the results shown in Tables II to VI. These experiments indicate the results obtained by varying the nickel content. In each case the precipitant was niobium silicide, and the carbon was maintained at less than 0.08%.

Table II HARDNESS VALUES FOR VARIOUS STAINLESS STEEL ALLOYS SOLUTION-TREATED AT 2200 F. AND AGED AT 600 F. FOR TIMES SHOWN Aging Time Composition As 30 1 hr. 5 10 quenched min. min. hrs. hrs.

18Cr-2Nb--4Sl l Rockwell C unless otherwise indicated.

Table III HARDNESS VALUES 1 FOR VARIOUS STAINLESS STEEL ALLOYS SOLUTION-TREATED AT 2200 F. AND AGED AT 800 F. FOR TIMES SHOWN Aging Time Composition As 10 30 1 5 10 50 113 quenched min. min. hr. hr. hr. hr. hr. hr.

180r-2Nb-4Si:

I Rockwell 0 unless otherwise indicated.

Hardness values, expressed as Rockwell B or Rockwell C when designated C.

PrecipitatlngAgent As Aging 10 30 1hr. 5 10 21 26-28 quenched Temp. mins. mins. hrs. hrs. hrs. hrs.

82 600 79 81 84 84 92 600 94 93 93 93 94 93 C24 600 C25 C26 C25 C27 C31 600 C30 C30 C31 C31 95 600 96 97 98 C26 600 C31 C29 C31 C82 C29 600 94 93 91 93 94 C22 600 023 C23 C23 C23 C21 83 800 83 86 85 86 87 87 92 800 92 94 96 97 97 C24 800 C23 C25 C25 C28 800 87 87 87 95 800 96 98 C26 800 O25 C27 C27 C28 C30 95 800 89 91 90 91 91 C22 800 O20 021 C21 C22 C23 1,000 90 90 91 93 C31 1, 000 C42 C50 C50 C53 C54 95 1, 000 98 100 99 100 92 1,000 93 94 93 96 97 C24 1, 000 C24 C30 C29 C33 C36 026 1,000 032 C34 O35 C36 C36 C36 90 1, 000 91 93 93 95 94 97 C22 1, 000 O23 C26 C27 C27 C29 82 1, 200 85 87 88 89 93 92 1, 200 94 95 96 98 100 C24 1, 200 C29 C32 C32 C31 C32 C31 1, 200 C44 O42 O42 O40 038 1,200 91 93 92 95 1, 200 98 100 99 026 1, 200 C32 C32 C32 C33 90 1,200 93 97 98 97 C22 1, 200 C31 C29 C30 C30 C31 nu,

Table IV HARDNESS VALUES 1 FOR VARIOUS STAINLESS STEEL ALLOYS SOLUTION TREATED AT 2200 F. AND AGED AT 1000 F. FOR TIMES SHOWN Aging Time Composition As 30 1 5 10 25 50 quenched min. min. hr hr. hr. hr. hr.

18Cr-2Nb-4Si:

4Ni 29 42 44 43 44 44 44 44 5N1- 29 40 41 41 41 42 41 42 6Ni- 32 46 45 48 46 45 44 44 7Ni- 35 48 48 47 46 46 44 45 1 Rockwell C unless otherwise indicated.

Table V HARDNESS VALUES 1 FOR VARIOUS STAINLESS STEEL ALLOYS SOLUTION TREATED AT 2200 F. AND AGED AT l Rockwell 0 unless otherwise indicated.

Table VI IIARDNESS VALUES FOR VARIOUS STAINLESS STEEL ALLOYS SOLUTION TREATED AT 2200 F. AND AGED AT 1400" F. FOR TIMES SHOWN Aging Time Composition As 10 30 1 5 10 25 93 quenched min. min. hr. hr. hr. hr. hr.

18Gr2Nb-4Si:

6Ni 32 35 36 36 43 49 51 48 7Ni 35 40 40 46 46 46 50 46 1 Rockwell C unless otherwise indicated.

We claim:

1. An age-hardened stainless steel, consisting essentially of 10-20% chromium, 0-l0% nickel, 020% manganese, where the sum of the percentage of nickel and half the percentage of manganese is not in excess of 10%, 1.04% niobium, 1.5-5% silicon, the sum of the niobium and silicon additions being from 3% to 7%, less than 0.08% carbon, less than 0.05% oxygen, less than 0.05% nitrogen and remainder substantially all iron and permissible alloying elements, said permissible alloying elements being less than 5%, said niobium and silicon being predominantly in the form of a niobium silicide, said steel having been age-hardened by heat treatment substantially to its peak hardness.

2. A composition as in claim 1. in which the ratio of niobium to silicon is from 1:3 to 2:1.

3. A composition as in claim 1 in which the niobium is between 1.5 and 3% and the silicon between 3-5%.

4. A composition as in claim 3 in which the ratio of niobium to silicon is about 1:2.

5. A composition as in claim 1 in which the steel has a predominantly ferritic structure.

6. A composition as in claim 1 in which the permissible alloying elements are selected from the group consisting of molybdenum, tungsten, vanadium and mixtures thereof.

7. A method of producing an age-hardened stainless steel essentially consisting of chromium 10-20%, 0-10% nickel, 0-20% manganese, where the sum of the percentage of nickel and half the percentage of manganese is not in excess of 10%, carbon less than 0.08%, oxygen less than 0.05 nitrogen less than 0.05 niobium 1.0-4%, silicon 1.5-5%, the sum of the niobium and silicon additions being from 3% to 7%, comprising the step of agehardeniug as-quenched steel for a time and at a temperature in the range of 800 F. to 1400 F. which will produce substantially a peak hardness.

References Cited in the file of this patent UNITED STATES PATENTS 2,156,307 Rapatz May 2, 1939 2,334,870 Franks Nov. 23, 1943 2,523,917 Payson Sept. 26, 1950 2,536,034- Clarke Jan. 2, 1951 2,824,795 Eberle Feb. 25, 1958 2,920,954 Mott Ian. 12, 1960 

1. AN AGE-HARDENED STAINLESS STEEL, CONSISTING ESSENTIALLY OF 10-20% CHROMIUM, 0-10% NICKEL, 0-20% MANGANESE, WHERE THE SUM OF THE PRECENTAGE OF NICKEL AND HALF THE PERCENTAGE OF MANGANESE IS NOT IN EXCESS OF 10%, 1.0-4% NIOBIUM, 1.5-5% SILICON, THE SUM OF THE NIOBIUM AND SILICON ADDITIONS BEING FROM 3% TO 7%, LESS THAN 0.08% CARBON, LESS THAN 0.05% OXYGEN, LESS THAN 0.05% NITROGEN AND REMAINDER SUBSTANTIALLY ALL IRON AND PERMISSIBLE ALLOYING ELEMENTS, SAID PERMISSIBLE ALLOYING ELEMENTS BEING LESS THAN 5%, SAID NIOBIUM AND SILICON BEING PREDOMINANTLY IN THE FORM OF A NIOBIUM SILICIDE, SAID STEEL HAVING BEEN AGE-HARDENED BY HEAT TREATMENT SUBSTANTIALLY TO ITS PEAK HARDNESS. 